Char layer

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

Intumescent additives. React with the polymer substrate to produce a char layer which forms an effective barrier between heat source and oxygen and... 

Gibov et al. (9) showed that combustion vapors and air could penetrate through a typical char layer. Capillarity served to bring molten polymer to the surface where it could pyrolyze and burn. One answer to this problem is obviously to create a closed cell foam. Gibov et al. showed that the incorporation of boric acid and ammonium phosphate helped minimize penetrability of the char (Fig. 1). 

The char layer from a burning polymer, while it exerts protective action, is itself vulnerable to oxidation. This can manifest itself either during flaming combustion as a constant destruction of the char as it forms, or as afterglow. Means for prevention of this undesired char destruction have been reported. In studies on preventing combustion of carbon fibers, incorporation of borates, phosphates, or low melting glasses has been shown to be effective (12, 13). 

Fig. 8.6 Reverse combustion (a) and forward combustion (b) in a fixed bed. Note that the unburnt fuel in (b) corresponds to the char layer of Fig. 8.5.

It is known that increased char yield is usually associated with improved flammability behavior ( 1). This can be understood if one considers that the volatile flammable products can only diffuse with difficulty through the char, and that the thermal conductivity of a porous char layer is relatively poor (2). The structure of the polymer can contribute to the amount of char formed based on the character of the functional groups present and the nature of the backbone (2,3). Ritchie ( ) found that for a series of unsaturated polyesters and their copolymers, the temperatures at which carbon dioxide is eliminated was in the range of 280 to 345°C depending on the structure of the polyester. Aliphatic polyesters and their copolymers have less thermal... 

Fluidized-bed combustion systems use a heated bed of sandlike material suspended (fluidized) within a rising column of turbulent air to burn many types and classes of waste fuels. The vendor claims that this technique results in improved combustion efficiency of high moisture content fuels and is adaptable to a variety of waste -type fuels. The scrubbing action of the bed material on the fuel particle is said to enhance the combustion process by stripping away the carbon dioxide and char layers that normally form around the fuel particle. This allows oxygen to reach the combustible material much more readily and increases the rate and efficiency of the combustion process. 

Figure 6.10. Accumulation of C in non-steady-state soils of a mature black spruce/moss forest in central Manitoba, Canada. Data shown are (A) for sphagnum moss that has accumulated since the site last burned (-100 yr before sampling), and (B) for the humus and charred layer below the regrowing moss and including the A horizon. The soil is developed on the sediments of a lake that dried up -7000 years ago. The parameters 7 = plant input (kgCuf2yr ) and k = decomposition constant (yr-1). Reprinted from Trumbore and Harden (1997), with permission from the American Geophysical Union.

F (927°C) at 1 hour, and 1,850°F (1,010°C) at 2 hours (31). When wood is exposed to these conditions, the first visual effect of thermal degradation (Figure 1) is indicated by browning of the wood at about 350°F to 400°F (175°C to 200°C). The temperature which characterized the base of the char layer was 550°F (288°C). After the first 1/4 inch of char development, the rate that this char layer moved into the solid wood—the rate of fire penetration—was about 38 millimeters per hour (1-1/2 in/hr). 

Figure 1. Fire penetration into wood and formation of char layer under the fire exposure conditions of...

Glowing is the visual evidence of combustion of the carbon in the char layer of the burning wood. If flaming of the released combustible gases has ceased, the glowing of the char is usually termed afterglow. 

FIGURE 6.28 SEM image of inner surface of char layer of the formulation without OLDH (a) and with 1.5 wt % OLDH (b). (From Wang, Z. et al., Prog. Org. Coating, 53, 29, 2005. With permission.)... 

The flame-retardant properties as evaluated by the cone calorimeter indicate that adding TPOSS to polycarbonate leads to a significant reduction in the PHRR value, but no improvement in the time to ignition was observed (Figure 8.12). The decrease in the time to ignition was attributed to the evolution of small molecules produced from dissociation of TPOSS. The authors proposed that during the combustion process a char layer is formed on the surface of the sample and TPOSS undergoes... 

Fire-retarded materials functioning in the condensed phase, such as intumescent systems, form, on heating, foamed cellular charred layers on the surface, which protects the underlying material from the action of the heat flux or the flame. It is recognized that the formation of the effective char occurs via a... 

Several micron-sized layered silicates, such as talcs, can improve the fire retarding behavior of EVA by partial substitution of metal hydroxides. Clerc et al.63 have shown that better fire performance was achieved using higher values of the lamellarity index and specific surface area for four different types of talcs in MH/EVA blends. Expanded mineral and charred layers were formed, similar to intumescent compositions with APP, proving the barrier effect on mass transfer, even at the micron scale for the mineral filler. 

A similar kind of synergy was investigated by Cui et al.,90 who prepared by melt blending nano-modified ATH using oxalic acid, a red phosphorus masterbatch and high impact polystyrene (HIPS). Unfortunately, the use of variable amounts of HIPS in the compositions has limited the possibility to see evidence of synergistic effects. The authors have stressed the well-developed and robust character of the char layer formed after UL-94V flame test for the composition HIPS/modified ATH/ Red phosphorus (68/20/12). The use of FTIR confirmed also that both P-O-P and P-O-C groups were present in the char. 

Since the majority of research carried out on flame retardancy of nanocomposite has dealt with OMLS, the most investigated combinations have concerned the corresponding class of nanocomposites of polymers, particularly EVA copolymer, PP, and polystyrene. The great interest taken in the development of IFR systems has also entailed the development of various and complex compositions in which OMLS have been associated with different intumescent systems containing APP and co-synergists able to promote the formation of a stable and expanded char layer reinforced by aluminophosphate species formed by reaction between APP and OMLS. 

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